A varactor is an electronic component with a capacitance that changes in response to an applied bias voltage. While there are many different types of varactors, an exemplary varactor diode 10 is shown in FIGS. 1A and 1B. The exemplary varactor diode 10 is operated by coupling the varactor diode 10 between a bias voltage VBIAS and ground. Specifically, the varactor diode 10 is coupled such that a cathode of the varactor diode 10 is coupled to the bias voltage VBIAS while an anode of the varactor diode 10 is coupled to ground. An input node 12A may be coupled to the cathode of the varactor diode 10, while an output node 12B may be coupled to the anode of the varactor diode 10. As the bias voltage VBIAS is changed, a capacitance CD between the input node 12A and the output node 12B also changes. This is due to the fact that the bias voltage VBIAS, which is a reverse-bias voltage, controls a width WDR of a depletion region within a P-N junction of the varactor diode 10, as shown in FIG. 1B. Specifically, the bias voltage VBIAS is directly proportional to the width WDR of the depletion region, such that as the bias voltage VBIAS increases, the width WDR of the depletion region also increases, and vice-versa. The width WDR of the depletion region is in turn inversely proportional to the capacitance CD across the varactor diode 10, such that as the width WDR of the depletion region increases, the capacitance CD across the varactor diode 10 decreases. Accordingly, the bias voltage VBIAS is able to control the capacitance CD across the varactor diode 10.
Varactors are used in a variety of different applications. For example, many varactors are currently used in radio frequency (RF) circuitry such as RF front-end circuitry. In such applications, a time-varying RF signal is generally applied across the varactor diode 10. The RF signal may modulate the capacitance CD of the varactor diode 10 due to the same mechanism of action described above with respect to the bias voltage VBIAS, which may be undesirable in many situations. In order to counteract this modulation effect, multiple varactor diodes 10 may be coupled in series between the input node 12A and the output node 12B, as shown in FIG. 2. Specifically, FIG. 2 shows a number of varactor diodes 10A-10H coupled in an alternating-polarity configuration such that a cathode of a first varactor diode 10A is coupled to the input node 12A, an anode of the first varactor diode 10A is coupled to an anode of a second varactor diode 10B, a cathode of the second varactor diode 10B is coupled to a cathode of a third varactor diode 10C, an anode of the third varactor diode 10C is coupled to an anode of a fourth varactor diode 10D, a cathode of the fourth varactor diode 10D is coupled to a cathode of a fifth varactor diode 10E, an anode of the fifth varactor diode 10E is coupled to an anode of a sixth varactor diode 10F, a cathode of the sixth varactor diode 10F is coupled to a cathode of a seventh varactor diode 10G, an anode of the seventh varactor diode 10G is coupled to an anode of an eighth varactor diode 10H, and a cathode of the eighth varactor diode 10H is coupled to the output node 12B. The bias voltage VBIAS is coupled to the cathode of the first varactor diode 10A via a first bias resistor RB1, coupled to the cathode of the second varactor diode 10B and the third varactor diode 10C via a second bias resistor RB2, coupled to the cathode of the fourth varactor diode 10D and the fifth varactor diode 10E via a third bias resistor RB3, and coupled to the cathode of the sixth varactor diode 10F and the seventh varactor diode 10G via a fourth bias resistor RB4. Further, the anode of the first varactor diode 10A and the second varactor diode 10B are coupled to ground via a fifth bias resistor RB5, the anode of the third varactor diode 10C and the fourth varactor diode 10D are coupled to ground via a sixth bias resistor RB6, the anode of the fifth varactor diode 10E and the sixth varactor diode 10F are coupled to ground via a seventh bias resistor RB7, and the anode of the seventh varactor diode 10G and the eighth varactor diode 10H are coupled to ground via an eighth bias resistor RB8. Because each one of the varactor diodes 10A-10H are essentially coupled in a reverse-bias configuration between the bias voltage VBIAS and ground, each one of the varactor diodes 10A-10H will vary the capacitance CD thereof in response to the bias voltage VBIAS as discussed above. Further, due to the fact that the varactor diodes 10A-10H are stacked in an alternating-polarity configuration, an increase in the capacitance of one of the diodes due to an RF signal placed across the varactor diodes 10A-10H is counteracted by a decrease in the capacitance of a corresponding reverse-connected varactor diode such that the net effect of an applied RF signal on the overall capacitance of the varactor diodes 10A-10H is minimal.
Generally, it is desirable for the capacitance of a varactor diode to change as quickly as possible in response to a change in the bias voltage VBIAS. The response over time of a resistor-capacitor (RC) circuit such as a varactor to a given voltage function is described by Equation (1):τ=RC  (1)where τ is the time constant of the circuit, R is a total resistance of the circuit as seen from the source of the voltage, and C is a total capacitance as seen from the source of the voltage. Higher values of τ are associated with an increased delay between an applied voltage and a change in the capacitance of the circuit. Accordingly, the larger the time constant associated with a varactor diode, the longer the time delay associated with a change in the bias voltage VBIAS and a corresponding change in the capacitance of the varactor diode.
In order to reduce the propagation of RF signals towards the bias voltage VBIAS and ground in the circuitry shown in FIG. 2, the bias resistors RB1-RB8 must generally be kept quite large, on the order of 20 kΩ and larger. Further, due to the fact that each one of the varactor diodes 10A-10H are essentially coupled in parallel between the bias voltage VBIAS and ground, the combined capacitance of the varactor diodes as seen by the bias voltage VBIAS is the sum of each one of the varactor diodes 10A-10H. Accordingly, the combination of the resistance and the capacitance RC seen by the bias voltage VBIAS is quite large, resulting in a large time constant τ, and thus a relatively slow response time of the varactor circuitry shown in FIG. 2.
Accordingly, FIG. 3 shows an alternative configuration for the varactor diodes 10A-10H. Specifically, FIG. 3 shows the varactor diodes coupled in series between the input node 12A and the output node 12B, such that the cathode of the first varactor diode 10A is coupled to the input node, the anode of the eighth varactor diode 10H is coupled to the output node 12B, and the remaining diodes are coupled between the first varactor diode 10A and the eighth varactor diode 10H in an anode-to-cathode configuration as shown. The bias voltage VBIAS is coupled to the cathode of the first varactor diode 10A via a first biasing resistor RB1 and coupled to the anode of the eighth varactor diode 10H via a second biasing resistor RB2. The resistance seen from the bias voltage VBIAS is thus significantly decreased compared to the circuitry shown in FIG. 2. Further, because the varactor diodes 10A-10C are coupled in series, the effective value thereof is calculated as shown in Equation (2):
                    1                              1                          C              1                                +                      1                          C              2                                +                                    1                              C                3                                      ⁢            …                                              (        2        )            
Accordingly, both the resistance and the capacitance RC associated with the circuitry shown in FIG. 3 is significantly reduced compared to that shown in FIG. 2, which results in significant reductions in the time constant τ thereof. Such a performance increase in the response time of the varactor diode circuitry shown in FIG. 3 comes at the expense of the ability of the circuitry to cancel the effects of RF signal modulation as in the alternating-polarity circuitry discussed above with respect to FIG. 2.
Conventionally, varactor diode structures such as those described above with respect to FIGS. 2 and 3 have been made by placing a number of co-planar varactor diodes 10A-10H next to one other on a substrate, as shown in FIG. 4. Each one of the diodes consumes area on the substrate, and thus the area required for the diodes may become quite large for circuitry requiring multiple stacked varactor diodes such as that shown in FIGS. 2 and 3. While a dual-stack varactor for alternating-polarity applications such as that shown in FIG. 2 has been described in co-pending and co-assigned U.S. patent application Ser. No. 14/273,316, the contents of which are hereby incorporated by reference in their entirety, series-coupled varactor diodes such as those shown in FIG. 2 have thus far been limited to single-stack solutions, resulting in the consumption of a relatively large area on a substrate.
Accordingly, there is a need for stacked varactor circuitry for series-connected varactor diodes with a reduced area.